Pistons of aluminum or cast iron are molded in a procedure which produces excess stock, including gates and sprues. After the piston has been molded, and in accordance with conventional practice, the sprues on the side of the piston are broken off, the gates on the end are sawed off, and the piston is then rough finish machined to remove the remaining excess stock. After rough finishing, the piston may be annealed and then finish machined. Rough finishing is done by rapidly rotating the piston and turning it against a single point tool on the side of the piston and also against a single point tool on the end of the piston. Rotation of the piston is sufficiently rapid to produce enough surface speed to generate the heat necessary to render the chips molten, so that less power is required. However, high speed rotation of the piston may cause it to expand by centrifugal force, and if the piston wall is of non-uniform thickness circumferentially, the piston will expand unevenly and become distorted. Distortion of the piston results in uneven stock removal.
This invention involves the rough finishing of a piston or the like, in which turning by a single point tool or tools is replaced by milling. A milling cutter rotated at high speed removes excess stock from a relatively slowly rotating piston. As with single point turning, high surface speed is necessary in order to produce molten chips. However, with the present invention in which a rapidly rotating milling cutter is employed, the piston needs to be turned only relatively slowly at speeds such that centrifugal force does not cause distortion. The sprues on the side of the piston are broken off before milling. However, the gates on the end of the piston are removed by the milling cutter. Thus this invention eliminates the necessity of sawing off the gates.
In accordance with the method and apparatus of this invention, the piston is rotated relatively slowly and a relatively rapidly rotating milling cutter is moved along the rotating piston in a manner such that the cutting elements of the milling cutter successively engage and remove stock from the surface of the piston. The milling cutter is guided along a path for rough finishing both the end and side surfaces of the piston.